Studies in rodent models suggest that bone marrow stem cells are capable of reconstituting a variety of tissues including not only mesenchymal tissues such as bone, cartilage, ligament, adipocytes, skeletal and cardiac muscles but also hepatocytes and CNS cells. This application pertains to studies of tissue reconstituting potential of human cord blood and adult stem cells based on a mouse/human xenograft model developed in our laboratory. In this model, "conditioned" newborn NOD/SCID/Beta2-microglobulin-null mice supported high-level hematopoietic engraftment by human cord blood cells. Recent studies using this model indicated that cord blood stem cells are capable of generating human hepatocytes and may also have the abilities to generate exocrine pancreatic cells and cardiomyocytes. This grant proposal is to determine the tissue reconstituting potential of human cord blood stem cells using this neonatal mouse xenograft model. Attempts will also be made to identify and purify the stem cells that are present in the cord blood by determining the surface phenotypes of tissue-engrafting cells. The major techniques to be used are fluorescence in situ hybridization (FISH) to identify human cells and immunohistochemistry, RT-PCR, and in situ hybridization for mRNA to delineate the tissue/cell lineages of the engrafted human cells. Comparison of these results with a separate study in our laboratory of tissue reconstituting potential of adult mouse stem cells indicates that there may be significant developmental control of the stem cell potentials. Based on these observations the following Specific Aims are proposed. Specific Aim #1: To determine the tissue reconstituting potential of human cord blood stem cells in a neonatal mouse xenograft model. Specific Aim #2: To identify and purify the human tissue reconstituting cells using the neonatal mouse xenograft model. Specific Aim #3: To determine the developmental control of the tissue reconstituting potentials of the stem cells in neonatal and adult mouse xenografts.